In a cellular radiotelephone system, the area served by the system is divided into geographically-defined cells. Each cell has a base station which serves MSs within its geographic area. An MS is linked to its serving cell base station, and it must also identify multiple neighbor cell base stations in order to facilitate reliable handover if the MS travels outside of the geographic range of its present serving cell.
FIG. 1 shows part of a cellular radiotelephone system that uses a seven-cell cell cluster. A different number of cells can be implemented in a cell cluster, and seven is merely chosen for explanation purposes. An MS 190 in cell 110 should be linked to the base station 180 in cell 110, thus the cell 110 is the MS 190 serving cell, and the serving cell base station should include surrounding cells 120, 130, 140, 150, 160, 170 in the serving cell's broadcast channel (BCH) allocation list. According to Global System for Mobile Communications (GSM) specifications, a BCH allocation list can include the BCH frequencies of up to thirty-two neighbor cells. The MS measures the power of each channel in the BCH allocation list and reports on up to six neighbor cells to the serving cell base station in a reporting table. Normally, these six channels in the reporting table are the six strongest channels from the BCH allocation list.
The GSM specifications for a digital radiotelephone system require that an MS decode the base station identification code (BSIC) of each channel in the reporting table at least once every ten seconds. An MS must complete two basic steps to decode a BSIC: (1) detect a frequency burst or frequency correction channel (FCH) on a cell's BCH to synchronize with the base station in the frequency domain (and pre-synchronize with the base station in the time domain); and (2) demodulate the synchronization burst or synchronization channel (SCH) of the cell's BCH to synchronize with the base station in the time domain. After the SCH has been demodulated, the mobile is fully synchronized to the base station and the BSIC is decoded.
FIG. 2 shows a BCH multiframe 200 according to GSM specifications. A BCH is broadcast by each base station and uses a repeating 51-frame structure with an FCH 210 occurring during frame numbers 0, 10, 20, 30, and 40 as shown. An SCH 220 occurs during frame numbers 1, 11, 21, 31, and 41 in a BCH multiframe 200 as shown in FIG. 2.
FIG. 3 shows a traffic channel (TCH) multiframe 300 according to GSM specifications. An MS uses a TCH to transmit user data, such as speech or computer data, to its serving cell base station. A TCH uses a repeating 26-frame structure with one idle frame 310 as the last frame in each TCH multiframe 300. The MS can detect an FCH, or demodulate an SCH, on a cell's BCH during the idle frame of a TCH multiframe 300.
FIG. 4 shows whether an idle frame of a TCH multiframe will intersect with an FCH, or an SCH, of a cell's BCH multiframe. Starting from frame 0 of the BCH multiframe 200 shown in FIG. 2 being aligned with frame 0 of the TCH multiframe 300 shown in FIG. 3, the idle frame 310 will first align with frame 25 of the BCH multiframe 200. Next, frame 0 of the TCH multiframe 300 aligns with frame 26 of the BCH multiframe 200, and the idle frame 310 will next align with the FCH 210 in frame 0 of the next BCH multiframe. This FCH is shown as FCH 410 in FIG. 4. The next idle frame aligns with a frame 26 of the BCH multiframe, and the fourth idle frame aligns with SCH 220 in frame 1 of the third BCH multiframe 200. This SCH is shown as SCH 420 in FIG. 4. The next six idle frames do not encounter either on FCH or an SCH. The alignment of the idle frames continues as shown in FIG. 4. As seen from FIG. 4, the worst case scenario for detecting an FCH burst on a cell's BCH can take up eleven idle frames. In other words, in the worst case scenario, ten idle frames are wasted. Thus, there is an opportunity to use unused frames in a more efficient manner to synchronize to a cell's base station.